Transitional Vertebrate Fossils FAQ
Part 1B

Transition from amphibians to amniotes (first reptiles)

The major functional difference between the ancient, large
amphibians and the first little reptiles is the amniotic egg.
Additional differences include stronger legs and girdles, different
vertebrae, and stronger jaw muscles. For more info, see Carroll
(1988) and Gauthier et al. (in Benton, 1988)

Solenodonsaurus (mid-Pennsylvanian) -- An incomplete
fossil, apparently between the anthracosaurs and the cotylosaurs.
Loss of palatal fangs, loss of lateral line on head, etc. Still
just a single sacral vertebra, though.

Hylonomus, Paleothyris (early Pennsylvanian) --
These are protorothyrids, very early cotylosaurs (primitive
reptiles). They were quite little, lizard-sized animals with
amphibian-like skulls (amphibian pineal opening, dermal bone,
etc.), shoulder, pelvis, & limbs, and intermediate teeth and
vertebrae. Rest of skeleton reptilian, with reptilian jaw muscle,
no palatal fangs, and spool-shaped vertebral centra. Probably no
eardrum yet. Many of these new "reptilian" features are also seen
in little amphibians (which also sometimes have
direct-developing eggs laid on land), so perhaps these features
just came along with the small body size of the first
reptiles.

The ancestral amphibians had a rather weak skull and paired
"aortas" (systemic arches). The first reptiles immediately split
into two major lines which modified these traits in different ways.
One line developed an aorta on the right side and strengthened the
skull by swinging the quadrate bone down and forward, resulting in
an enormous otic notch (and allowed the later development of good
hearing without much further modification). This group further
split into three major groups, easily recognizable by the number of
holes or "fenestrae" in the side of the skull: the anapsids (no
fenestrae), which produced the turtles; the diapsids (two
fenestrae), which produced the dinosaurs and birds; and an offshoot
group, the eurapsids (two fenestrae fused into one), which produced
the ichthyosaurs.

The other major line of reptiles developed an aorta on left side
only, and strengthened the skull by moving the quadrate bone up and
back, obliterating the otic notch (making involvement of the jaw
essential in the later development of good hearing). They developed
a single fenestra per side. This group was the synapsid reptiles.
They took a radically different path than the other reptiles,
involving homeothermy, a larger brain, better hearing and more
efficient teeth. One group of synapsids called the "therapsids"
took these changes particularly far, and apparently produced the
mammals.

Some transitions among reptiles

I will review just a couple of the reptile phylogenies, since
there are so many.... Early reptiles to turtles: (Also see Gaffney
& Meylan, in Benton 1988)

Here we come to a controversy; there are two related groups of
early anapsids, both descended from the captorhinids, that could
have been ancestral to turtles. Reisz & Laurin (1991, 1993)
believe the turtles descended from procolophonids, late Permian
anapsids that had various turtle-like skull features. Others,
particularly Lee (1993) think the turtle ancestors are pareiasaurs:

Scutosaurus and other pareiasaurs (mid-Permian) -- Large
bulky herbivorous reptiles with turtle-like skull features. Several
genera had bony plates in the skin, possibly the first signs of a
turtle shell.

Deltavjatia vjatkensis (Permian) -- A recently
discovered pareiasaur with numerous turtle-like skull features
(e.g., a very high palate), limbs, and girdles, and lateral
projections flaring out some of the vertebrae in a very shell-like
way. (Lee, 1993)

Proganochelys (late Triassic) -- a primitive turtle,
with a fully turtle-like skull, beak, and shell, but with some
primitive traits such as rows of little palatal teeth, a
still-recognizable clavicle, a simple captorhinid-type jaw
musculature, a primitive captorhinid- type ear, a non-retractable
neck, etc..

Recently discovered turtles from the early Jurassic, not yet
described.

Mid-Jurassic turtles had already divided into the two main
groups of modern turtles, the side-necked turtles and the
arch-necked turtles. Obviously these two groups developed neck
retraction separately, and came up with totally different
solutions. In fact the first known arch-necked turtles, from the
Late Jurassic, could not retract their necks, and only later did
their descendents develop the archable neck. Early reptiles to
diapsids: (see Evans, in Benton 1988, for more info)

Petrolacosaurus, Araeoscelis (late Pennsylvanian)
-- First known diapsids. Both temporal fenestra now present. No
significant change in jaw muscles. Have Hylonomus-style teeth, with
many small marginal teeth & two slightly larger canines. Still
no eardrum.

Claudiosaurus (late Permian) -- An early diapsid with
several neodiapsid traits, but still had primitive cervical
vertebrae & unossified sternum. probably close to the ancestry
of all diapsides (the lizards & snakes & crocs &
birds).

Planocephalosaurus(early Triassic) -- Further along the
line that produced the lizards and snakes. Loss of some skull
bones, teeth, toe bones.

Protorosaurus, Prolacerta (early Triassic) --
Possibly among the very first archosaurs, the line that produced
dinos, crocs, and birds. May be "cousins" to the archosaurs,
though.

Proterosuchus (early Triassic) -- First known
archosaur.

Hyperodapedon, Trilophosaurus (late Triassic) --
Early archosaurs.

Some species-to-species transitions:

De Ricqles (in Chaline, 1983) documents several possible cases
of gradual evolution (also well as some lineages that showed abrupt
appearance or stasis) among the early Permian reptile genera
Captorhinus, Protocaptorhinus, Eocaptorhinus,
and Romeria.

Horner et al. (1992) recently found many excellent transitional
dinosaur fossils from a site in Montana that was a coastal plain in
the late Cretaceous. They include:

Many transitional ceratopsids between Styracosaurus and
Pachyrhinosaurus

Many transitional lambeosaurids (50! specimens) between
Lambeosaurus and Hypacrosaurus.

A transitional pachycephalosaurid between Stegoceras and
Pachycephalosaurus

A transitional tyrannosaurid between Tyrannosaurus and
Daspletosaurus.

All of these transitional animals lived during the same brief
500,000 years. Before this site was studied, these dinosaur groups
were known from the much larger Judith River Formation, where the
fossils showed 5 million years of evolutionary stasis, following by
the apparently abrupt appearance of the new forms. It turns out
that the sea level rose during that 500,000 years, temporarily
burying the Judith River Formation under water, and forcing the
dinosaur populations into smaller areas such as the site in
Montana. While the populations were isolated in this smaller area,
they underwent rapid evolution. When sea level fell again, the new
forms spread out to the re-exposed Judith River landscape, thus
appearing "suddenly" in the Judith River fossils, with the
transitional fossils only existing in the Montana site. This is an
excellent example of punctuated equilibrium (yes, 500,000 years is
very brief and counts as a "punctuation"), and is a good example of
why transitional fossils may only exist in a small area, with the
new species appearing "suddenly" in other areas. (Horner et al.,
1992) Also note the discovery of Ianthosaurus, a genus that
links the two synapsid families Ophiacodontidae and Edaphosauridae.
(see Carroll, 1988, p. 367)

Transition from synapsid reptiles to mammals

This is the best-documented transition between vertebrate
classes. So far this series is known only as a series of genera or
families; the transitions from species to species are not
known. But the family sequence is quite complete. Each group is
clearly related to both the group that came before, and the group
that came after, and yet the sequence is so long that the fossils
at the end are astoundingly different from those at the beginning.
As Rowe recently said about this transition (in Szalay et al.,
1993), "When sampling artifact is removed and all available
character data analyzed [with computer phylogeny programs that do
not assume anything about evolution], a highly corroborated, stable
phylogeny remains, which is largely consistent with the temporal
distributions of taxa recorded in the fossil record." Similarly,
Gingerich has stated (1977) "While living mammals are well
separated from other groups of animals today, the fossil record
clearly shows their origin from a reptilian stock and permits one
to trace the origin and radiation of mammals in considerable
detail." For more details, see Kermack's superb and readable little
book (1984), Kemp's more detailed but older book (1982), and read
Szalay et al.'s recent collection of review articles (1993, vol.
1).

This list starts with pelycosaurs (early synapsid reptiles) and
continues with therapsids and cynodonts up to the first unarguable
"mammal". Most of the changes in this transition involved elaborate
repackaging of an expanded brain and special sense organs,
remodeling of the jaws & teeth for more efficient eating, and
changes in the limbs & vertebrae related to active,
legs-under-the-body locomotion. Here are some differences to keep
an eye on:

#

Early Reptiles

Mammals

1

No fenestrae in skull

Massive fenestra exposes all of braincase

2

Braincase attached loosely

Braincase attached firmly to skull

3

No secondary palate

Complete bony secondary palate

4

Undifferentiated dentition

Incisors, canines, premolars, molars

5

Cheek teeth uncrowned points

Cheek teeth (PM & M) crowned & cusped

6

Teeth replaced continuously

Teeth replaced once at most

7

Teeth with single root

Molars double-rooted

8

Jaw joint quadrate-articular

Jaw joint dentary-squamosal (*)

9

Lower jaw of several bones

Lower jaw of dentary bone only

10

Single ear bone (stapes)

Three ear bones (stapes, incus, malleus)

11

Joined external nares

Separate external nares

12

Single occipital condyle

Double occipital condyle

13

Long cervical ribs

Cervical ribs tiny, fused to vertebrae

14

Lumbar region with ribs

Lumbar region rib-free

15

No diaphragm

Diaphragm

16

Limbs sprawled out from body

Limbs under body

17

Scapula simple

Scapula with big spine for muscles

18

Pelvic bones unfused

Pelvis fused

19

Two sacral (hip) vertebrae

Three or more sacral vertebrae

20

Toe bone #'s 2-3-4-5-4

Toe bones 2-3-3-3-3

21

Body temperature variable

Body temperature constant

(*) The presence of a dentary-squamosal jaw joint has
been arbitrarily selected as the defining trait of a
mammal.

Paleothyris (early Pennsylvanian) -- An early
captorhinomorph reptile, with no temporal fenestrae at all.

Protoclepsydrops haplous (early Pennsylvanian) -- The
earliest known synapsid reptile. Little temporal fenestra, with all
surrounding bones intact. Fragmentary. Had amphibian-type vertebrae
with tiny neural processes. (reptiles had only just separated from
the amphibians)

Clepsydrops (early Pennsylvanian) -- The second earliest
known synapsid. These early, very primitive synapsids are a
primitive group of pelycosaurs collectively called
"ophiacodonts".

Archaeothyris (early-mid Pennsylvanian) -- A slightly
later ophiacodont. Small temporal fenestra, now with some reduced
bones (supratemporal). Braincase still just loosely attached to
skull. Slight hint of different tooth types. Still has some
extremely primitive, amphibian/captorhinid features in the jaw,
foot, and skull. Limbs, posture, etc. typically reptilian, though
the ilium (major hip bone) was slightly enlarged.

Varanops (early Permian) -- Temporal fenestra further
enlarged. Braincase floor shows first mammalian tendencies &
first signs of stronger attachment to rest of skull (occiput more
strongly attached). Lower jaw shows first changes in jaw
musculature (slight coronoid eminence). Body narrower, deeper:
vertebral column more strongly constructed. Ilium further enlarged,
lower-limb musculature starts to change (prominent fourth
trochanter on femur). This animal was more mobile and active. Too
late to be a true ancestor, and must be a "cousin".

Haptodus (late Pennsylvanian) -- One of the first known
sphenacodonts, showing the initiation of sphenacodont features
while retaining many primitive features of the ophiacodonts.
Occiput still more strongly attached to the braincase. Teeth become
size-differentiated, with biggest teeth in canine region and fewer
teeth overall. Stronger jaw muscles. Vertebrae parts & joints
more mammalian. Neural spines on vertebrae longer. Hip strengthened
by fusing to three sacral vertebrae instead of just two. Limbs very
well developed.

Dimetrodon, Sphenacodon or a similar sphenacodont
(late Pennsylvanian to early Permian, 270 Ma) -- More advanced
pelycosaurs, clearly closely related to the first therapsids
(next). Dimetrodon is almost definitely a "cousin" and not a
direct ancestor, but as it is known from very complete fossils,
it's a good model for sphenacodont anatomy. Medium-sized fenestra.
Teeth further differentiated, with small incisors, two huge deep-
rooted upper canines on each side, followed by smaller cheek teeth,
all replaced continuously. Fully reptilian jaw hinge. Lower jaw
bone made of multiple bones & with first signs of a bony prong
later involved in the eardrum, but there was no eardrum yet, so
these reptiles could only hear ground-borne vibrations (they did
have a reptilian middle ear). Vertebrae had still longer neural
spines (spectacularly so in Dimetrodon, which had a sail),
and longer transverse spines for stronger locomotion muscles.

Biarmosuchia (late Permian) -- A therocephalian -- one
of the earliest, most primitive therapsids. Several primitive,
sphenacodontid features retained: jaw muscles inside the skull,
platelike occiput, palatal teeth. New features: Temporal fenestra
further enlarged, occupying virtually all of the cheek, with the
supratemporal bone completely gone. Occipital plate slanted
slightly backwards rather than forwards as in pelycosaurs, and
attached still more strongly to the braincase. Upper jaw bone
(maxillary) expanded to separate lacrymal from nasal bones,
intermediate between early reptiles and later mammals. Still no
secondary palate, but the vomer bones of the palate
developed a backward extension below the palatine bones. This is
the first step toward a secondary palate, and with exactly the same
pattern seen in cynodonts. Canine teeth larger, dominating the
dentition. Variable tooth replacement: some therocephalians (e.g
Scylacosaurus) had just one canine, like mammals, and
stopped replacing the canine after reaching adult size. Jaw hinge
more mammalian in position and shape, jaw musculature stronger
(especially the mammalian jaw muscle). The amphibian-like hinged
upper jaw finally became immovable. Vertebrae still
sphenacodontid-like. Radical alteration in the method of
locomotion, with a much more mobile forelimb, more upright
hindlimb, & more mammalian femur & pelvis. Primitive
sphenacodontid humerus. The toes were approaching equal length, as
in mammals, with #toe bones varying from reptilian to mammalian.
The neck & tail vertebrae became distinctly different from
trunk vertebrae. Probably had an eardrum in the lower jaw, by the
jaw hinge.

Procynosuchus (latest Permian) -- The first known
cynodont -- a famous group of very mammal-like therapsid reptiles,
sometimes considered to be the first mammals. Probably arose from
the therocephalians, judging from the distinctive secondary palate
and numerous other skull characters. Enormous temporal fossae for
very strong jaw muscles, formed by just one of the reptilian jaw
muscles, which has now become the mammalian masseter. The large
fossae is now bounded only by the thin zygomatic arch (cheekbone to
you & me). Secondary palate now composed mainly of palatine
bones (mammalian), rather than vomers and maxilla as in older
forms; it's still only a partial bony palate (completed in life
with soft tissue). Lower incisor teeth was reduced to four (per
side), instead of the previous six (early mammals had three).
Dentary now is 3/4 of lower jaw; the other bones are now a small
complex near the jaw hinge. Jaw hinge still reptilian. Vertebral
column starts to look mammalian: first two vertebrae modified for
head movements, and lumbar vertebrae start to lose ribs, the first
sign of functional division into thoracic and lumbar regions.
Scapula beginning to change shape. Further enlargement of the ilium
and reduction of the pubis in the hip. A diaphragm may have been
present.

Dvinia [also "Permocynodon"] (latest Permian) -- Another
early cynodont. First signs of teeth that are more than simple
stabbing points -- cheek teeth develop a tiny cusp. The temporal
fenestra increased still further. Various changes in the floor of
the braincase; enlarged brain. The dentary bone was now the major
bone of the lower jaw. The other jaw bones that had been present in
early reptiles were reduced to a complex of smaller bones near the
jaw hinge. Single occipital condyle splitting into two surfaces.
The postcranial skeleton of Dvinia is virtually unknown and it is
not therefore certain whether the typical features found at the
next level had already evolved by this one. Metabolic rate was
probably increased, at least approaching homeothermy.

Thrinaxodon (early Triassic) -- A more advanced
"galesaurid" cynodont. Further development of several of the
cynodont features seen already. Temporal fenestra still larger,
larger jaw muscle attachments. Bony secondary palate almost
complete. Functional division of teeth: incisors (four uppers and
three lowers), canines, and then 7-9 cheek teeth with cusps for
chewing. The cheek teeth were all alike, though (no premolars &
molars), did not occlude together, were all single- rooted, and
were replaced throughout life in alternate waves. Dentary still
larger, with the little quadrate and articular bones were loosely
attached. The stapes now touched the inner side of the quadrate.
First sign of the mammalian jaw hinge, a ligamentous connection
between the lower jaw and the squamosal bone of the skull. The
occipital condyle is now two slightly separated surfaces, though
not separated as far as the mammalian double condyles. Vertebral
connections more mammalian, and lumbar ribs reduced. Scapula shows
development of a new mammalian shoulder muscle. Ilium increased
again, and all four legs fully upright, not sprawling. Tail short,
as is necessary for agile quadrupedal locomotion. The whole
locomotion was more agile. Number of toe bones is 2.3.4.4.3,
intermediate between reptile number (2.3.4.5.4) and mammalian
(2.3.3.3.3), and the "extra" toe bones were tiny. Nearly complete
skeletons of these animals have been found curled up - a possible
reaction to conserve heat, indicating possible endothermy? Adults
and juveniles have been found together, possibly a sign of parental
care. The specialization of the lumbar area (e.g. reduction of
ribs) is indicative of the presence of a diaphragm, needed for
higher O2 intake and homeothermy. NOTE on hearing: The eardrum had
developed in the only place available for it -- the lower
jaw, right near the jaw hinge, supported by a wide prong (reflected
lamina) of the angular bone. These animals could now hear airborne
sound, transmitted through the eardrum to two small lower jaw
bones, the articular and the quadrate, which contacted the stapes
in the skull, which contacted the cochlea. Rather a roundabout
system and sensitive to low-frequency sound only, but better than
no eardrum at all! Cynodonts developed quite loose quadrates and
articulars that could vibrate freely for sound transmittal while
still functioning as a jaw joint, strengthened by the mammalian jaw
joint right next to it. All early mammals from the Lower Jurassic
have this low-frequency ear and a double jaw joint. By the middle
Jurassic, mammals lost the reptilian joint (though it still occurs
briefly in embryos) and the two bones moved into the nearby middle
ear, became smaller, and became much more sensitive to
high-frequency sounds.

Cynognathus (early Triassic, 240 Ma; suspected to have
existed even earlier) -- We're now at advanced cynodont level.
Temporal fenestra larger. Teeth differentiating further; cheek
teeth with cusps met in true occlusion for slicing up food, rate of
replacement reduced, with mammalian-style tooth roots (though
single roots). Dentary still larger, forming 90% of the
muscle-bearing part of the lower jaw. TWO JAW JOINTS in place,
mammalian and reptilian: A new bony jaw joint existed between the
squamosal (skull) and the surangular bone (lower jaw), while the
other jaw joint bones were reduced to a compound rod lying in a
trough in the dentary, close to the middle ear. Ribs more
mammalian. Scapula halfway to the mammalian condition. Limbs were
held under body. There is possible evidence for fur in fossil
pawprints.

Probainognathus (mid-Triassic, 239-235 Ma, Argentina) --
Larger brain with various skull changes: pineal foramen ("third
eye") closes, fusion of some skull plates. Cheekbone slender, low
down on the side of the eye socket. Postorbital bar still there.
Additional cusps on cheek teeth. Still two jaw joints. Still had
cervical ribs & lumbar ribs, but they were very short.
Reptilian "costal plates" on thoracic ribs mostly lost. Mammalian
#toe bones.

Exaeretodon (mid-late Triassic, 239Ma, South America) --
(Formerly lumped with the herbivorous gomphodont cynodonts.)
Mammalian jaw prong forms, related to eardrum support. Three
incisors only (mammalian). Costal plates completely lost. More
mammalian hip related to having limbs under the body. Possibly the
first steps toward coupling of locomotion & breathing. This is
probably a "cousin" fossil not directly ancestral, as it has
several new but non-mammalian teeth traits.

GAP of about 30 my in the late Triassic, from about 239-208 Ma.
Only one early mammal fossil is known from this time. The next time
fossils are found in any abundance, tritylodontids and
trithelodontids had already appeared, leading to some very heated
controversy about their relative placement in the chain to mammals.
Recent discoveries seem to show trithelodontids to be more mammal-
like, with tritylodontids possibly being an offshoot group (see
Hopson 1991, Rowe 1988, Wible 1991, and Shubin et al. 1991). Bear
in mind that both these groups were almost fully mammalian in every
feature, lacking only the final changes in the jaw joint and middle
ear.

Oligokyphus, Kayentatherium (early Jurassic, 208
Ma) -- These are tritylodontids, an advanced cynodont group. Face
more mammalian, with changes around eyesocket and cheekbone. Full
bony secondary palate. Alternate tooth replacement with
double-rooted cheek teeth, but without mammalian-style tooth
occlusion (which some earlier cynodonts already had). Skeleton
strikingly like egg- laying mammals (monotremes). Double jaw joint.
More flexible neck, with mammalian atlas & axis and double
occipital condyle. Tail vertebrae simpler, like mammals. Scapula is
now substantially mammalian, and the forelimb is carried directly
under the body. Various changes in the pelvis bones and hind limb
muscles; this animal's limb musculature and locomotion were
virtually fully mammalian. Probably cousin fossils (?), with
Oligokyphus being more primitive than Kayentatherium.
Thought to have diverged from the trithelodontids during that gap
in the late Triassic. There is disagreement about whether the
tritylodontids were ancestral to mammals (presumably during the
late Triassic gap) or whether they are a specialized offshoot group
not directly ancestral to mammals.

Pachygenelus, Diarthrognathus (earliest Jurassic,
209 Ma) -- These are trithelodontids, a slightly different advanced
cynodont group. New discoveries (Shubin et al., 1991) show that
these animals are very close to the ancestry of mammals. Inflation
of nasal cavity, establishment of Eustachian tubes between ear and
pharynx, loss of postorbital bar. Alternate replacement of mostly
single- rooted teeth. This group also began to develop double tooth
roots -- in Pachygenelus the single root of the cheek teeth
begins to split in two at the base. Pachygenelus also has
mammalian tooth enamel, and mammalian tooth occlusion. Double jaw
joint, with the second joint now a dentary-squamosal
(instead of surangular), fully mammalian. Incipient dentary
condyle. Reptilian jaw joint still present but functioning almost
entirely in hearing; postdentary bones further reduced to tiny rod
of bones in jaw near middle ear; probably could hear high
frequencies now. More mammalian neck vertebrae for a flexible neck.
Hip more mammalian, with a very mammalian iliac blade & femur.
Highly mobile, mammalian-style shoulder. Probably had coupled
locomotion & breathing. These are probably "cousin" fossils,
not directly ancestral (the true ancestor is thought to have
occurred during that late Triassic gap). Pachygenelus is
pretty close, though.

Adelobasileus cromptoni (late Triassic; 225 Ma, west
Texas) -- A recently discovered fossil proto-mammal from right in
the middle of that late Triassic gap! Currently the oldest known
"mammal." Only the skull was found. "Some cranial features of
Adelobasileus, such as the incipient promontorium housing
the cochlea, represent an intermediate stage of the character
transformation from non-mammalian cynodonts to Liassic mammals"
(Lucas & Luo, 1993). This fossil was found from a band of
strata in the western U.S. that had not previously been studied for
early mammals. Also note that this fossil dates from slightly
before the known tritylodonts and trithelodonts, though it
has long been suspected that tritilodonts and trithelodonts were
already around by then. Adelobasileus is thought to have
split off from either a trityl. or a trithel., and is either
identical to or closely related to the common ancestor of all
mammals.

Sinoconodon (early Jurassic, 208 Ma) -- The next known
very ancient proto-mammal. Eyesocket fully mammalian now (closed
medial wall). Hindbrain expanded. Permanent cheekteeth, like
mammals, but the other teeth were still replaced several times.
Mammalian jaw joint stronger, with large dentary condyle fitting
into a distinct fossa on the squamosal. This final refinement of
the joint automatically makes this animal a true "mammal".
Reptilian jaw joint still present, though tiny.

Kuehneotherium (early Jurassic, about 205 Ma) -- A
slightly later proto-mammal, sometimes considered the first known
pantothere (primitive placental-type mammal). Teeth and skull like
a placental mammal. The three major cusps on the upper & lower
molars were rotated to form interlocking shearing triangles as in
the more advanced placental mammals & marsupials. Still has a
double jaw joint, though.

Eozostrodon, Morganucodon, Haldanodon
(early Jurassic, ~205 Ma) -- A group of early proto-mammals called
"morganucodonts". The restructuring of the secondary palate and the
floor of the braincase had continued, and was now very mammalian.
Truly mammalian teeth: the cheek teeth were finally differentiated
into simple premolars and more complex molars, and teeth were
replaced only once. Triangular- cusped molars. Reversal of the
previous trend toward reduced incisors, with lower incisors
increasing to four. Tiny remnant of the reptilian jaw joint. Once
thought to be ancestral to monotremes only, but now thought to be
ancestral to all three groups of modern mammals -- monotremes,
marsupials, and placentals.

Peramus (late Jurassic, about 155 Ma) -- A
"eupantothere" (more advanced placental-type mammal). The closest
known relative of the placentals & marsupials. Triconodont
molar has with more defined cusps. This fossil is known only from
teeth, but judging from closely related eupantotheres (e.g.
Amphitherium) it had finally lost the reptilian jaw joint,
attaing a fully mammalian three-boned middle ear with excellent
high-frequency hearing. Has only 8 cheek teeth, less than other
eupantotheres and close to the 7 of the first placental mammals.
Also has a large talonid on its "tribosphenic" molars, almost as
large as that of the first placentals -- the first development of
grinding capability.

Endotherium (very latest Jurassic, 147 Ma) -- An
advanced eupantothere. Fully tribosphenic molars with a well-
developed talonid. Known only from one specimen. From Asia; recent
fossil finds in Asia suggest that the tribosphenic molar evolved
there.

Kielantherium and Aegialodon (early Cretaceous)
-- More advanced eupantotheres known only from teeth.
Kielantherium is from Asia and is known from slightly older
strata than the European Aegialodon. Both have the talonid
on the lower molars. The wear on it indicates that a major new
cusp, the protocone, had evolved on the upper molars. By the Middle
Cretaceous, animals with the new tribosphenic molar had spread into
North America too (North America was still connected to
Europe.)

Steropodon galmani (early Cretaceous) -- The first known
definite monotreme, discovered in 1985.

Pariadens kirklandi (late Cretaceous, about 95 Ma) --
The first definite marsupial. Known only from teeth.

Kennalestes and Asioryctes (late Cretaceous,
Mongolia) -- Small, slender animals; eyesocket open behind; simple
ring to support eardrum; primitive placental-type brain with large
olfactory bulbs; basic primitive tribosphenic tooth pattern. Canine
now double rooted. Still just a trace of a non-dentary bone, the
coronoid, on the otherwise all-dentary jaw. "Could have given rise
to nearly all subsequent placentals." says Carroll (1988).

Cimolestes, Procerberus, Gypsonictops
(very late Cretaceous) -- Primitive North American placentals with
same basic tooth pattern.

So, by the late Cretaceous the three groups of modern mammals
were in place: monotremes, marsupials, and placentals. Placentals
appear to have arisen in East Asia and spread to the Americas by
the end of the Cretaceous. In the latest Cretaceous, placentals and
marsupials had started to diversify a bit, and after the dinosaurs
died out, in the Paleocene, this diversification accelerated. For
instance, in the mid- Paleocene the placental fossils include a
very primitive primate-like animal (Purgatorius - known only
from a tooth, though, and may actually be an early ungulate), a
herbivore-like jaw with molars that have flatter tops for better
grinding (Protungulatum, probably an early ungulate), and an
insectivore (Paranyctoides).

The decision as to which was the first mammal is somewhat
subjective. We are placing an inflexible classification system on a
gradational series. What happened was that an intermediate group
evolved from the 'true' reptiles, which gradually acquired
mammalian characters until a point was reached where we have
artificially drawn a line between reptiles and mammals. For
instance, Pachygenulus and Kayentatherium are both
far more mammal-like than reptile-like, but they are both called
"reptiles".

Transition from diapsid reptiles to birds

In the mid-1800's, this was one of the most significant gaps in
vertebrate fossil evolution. No transitional fossils at all were
known, and the two groups seemed impossibly different. Then the
exciting discovery of Archeopteryx in 1861 showed clearly
that the two groups were in fact related. Since then, some other
reptile-bird links have been found. On the whole, though, this is
still a gappy transition, consisting of a very large-scale series
of "cousin" fossils. I have not included Mononychus (as it
appears to be a digger, not a flier, well off the line to modern
birds). See Feduccia (1980) and Rayner (1989) for more discussion
of the evolution of flight, and Chris Nedin's excellent
Archeopteryx FAQ
for more info on that critter.

Coelophysis (late Triassic) -- One of the first theropod
dinosaurs. Theropods in general show clear general skeletal
affinities with birds (long limbs, hollow bones, foot with 3 toes
in front and 1 reversed toe behind, long ilium). Jurassic theropods
like Compsognathus are particularly similar to birds.

Deinonychus, Oviraptor, and other advanced
theropods (late Jurassic, Cretaceous) -- Predatory bipedal advanced
theropods, larger, with more bird-like skeletal features:
semilunate carpal, bony sternum, long arms, reversed pubis. Clearly
runners, though, not fliers. These advanced theropods even had
clavicles, sometimes fused as in birds. Says Clark (1992): "The
detailed similarity between birds and theropod dinosaurs such as
Deinonychus is so striking and so pervasive throughout the
skeleton that a considerable amount of special pleading is needed
to come to any conclusion other than that the sister-group of birds
among fossils is one of several theropod dinosaurs." The particular
fossils listed here are are not directly ancestral, though,
as they occur after Archeopteryx.

Lisboasaurus estesi & other "troodontid
dinosaur-birds" (mid-Jurassic) -- A bird-like theropod reptile with
very bird-like teeth (that is, teeth very like those of early
toothed birds, since modern birds have no teeth). These really
could be ancestral.

GAP: The exact reptilian ancestor of Archeopteryx, and
the first development of feathers, are unknown. Early bird
evolution seems to have involved little forest climbers and then
little forest fliers, both of which are guaranteed to leave very
bad fossil records (little animal + acidic forest soil = no
remains). Archeopteryx itself is really about the best we
could ask for: several specimens has superb feather impressions, it
is clearly related to both reptiles and birds, and it
clearly shows that the transition is feasible.

One possible ancestor of Archeopteryx is
Protoavis (Triassic, ~225 Ma) -- A highly controversial
fossil that may or may not be an extremely early bird.
Unfortunately, not enough of the fossil was recovered to determine
if it is definitely related to the birds.

Archeopteryx lithographica (Late Jurassic, 150 Ma) --
The several known specimes of this deservedly famous fossil show a
mosaic of reptilian and avian features, with the reptilian features
predominating. The skull and skeleton are basically reptilian
(skull, teeth, vertebrae, sternum, ribs, pelvis, tail, digits,
claws, generally unfused bones). Bird traits are limited to an
avian furcula (wishbone, for attachment of flight muscles; recall
that at least some dinosaurs had this too), modified forelimbs, and
-- the real kicker -- unmistakable lift-producing flight feathers.
Archeopteryx could probably flap from tree to tree, but
couldn't take off from the ground, since it lacked a keeled
breastbone for large flight muscles, and had a weak shoulder
compared to modern birds. May not have been the direct ancestor of
modern birds. (Wellnhofer, 1993)

"Las Hoyas bird" or "Spanish bird" [not yet named; early
Cretaceous, 131 Ma) -- Another recently found "little forest
flier". It still has reptilian pelvis & legs, with bird-like
shoulder. Tail is medium-length with a fused tip. A fossil down
feather was found with the Las Hoyas bird, indicating homeothermy.
(Sanz et al., 1992)

Ambiortus dementjevi (early Cretaceous, 125 Ma) -- The
third known "little forest flier", found in 1985. Very fragmentary
fossil.

Hesperornis, Ichthyornis, and other Cretaceous
diving birds -- This line of birds became specialized for diving,
like modern cormorants. As they lived along saltwater coasts, there
are many fossils known. Skeleton further modified for flight
(fusion of pelvis bones, fusion of hand bones, short & fused
tail). Still had true socketed teeth, a reptilian trait.

[Note: a classic study of chicken embryos showed that chicken
bills can be induced to develop teeth, indicating that chickens
(and perhaps other modern birds) still retain the genes for making
teeth. Also note that molecular data shows that crocodiles are
birds' closest living relatives.]